In general, spread spectrum communication systems employ a method of communication wherein the transmission bandwidth is broader or wider than the information bandwidth but is not a function of the information bandwidth. Two articles of background interest here which discuss the nature and development of prior art spread spectrum communication systems are: Scholtz, "The Origins of Spread Spectrum Communications", IEEE Transactions on Communications, Vol COM 30, No. 5, May 1982; and Utlaut, "Spread Spectrum: Principles and Possible Application to Spectrum Utilization and Allocation", ITU Telecommunication Journal, Vol. 45, pp. 20-32, Jan. 1978.
Spread spectrum communication systems can generally be broken down into three basic types: (1) direct sequence systems; (2) frequency hopping systems; and (3) hybrid system. Broadly speaking, direct sequence (DS) spread spectrum communication systems involve multiplication of the information to be transmitted by a spreading code and transmitting the resultant product. On the other hand, frequency hopping (FH) spread spectrum communication systems provide for random selection of a carrier frequency using a random code. Hybrid systems, as the name implies, combine aspects of both DS and FH spread spectrum communication systems.
As will appear, the present invention concerns improvements in receivers for frequency hopping and hybrid systems. For hybrid systems incorporating this improvement, it is assumed that the spreading code for the direct sequence is known, i.e., the spreading code must be in phase with the code in the transmitted signal as received at the receiver. This information also must be known for frequency hopping systems although the code can be different. In other words, the invention is an improvement on systems wherein the receiver knows, and is in synchronism with, the direct sequence spread spectrum code. On the other hand, as will also appear, among other advantages of the invention is the advantage that knowledge of the hopping code is not required.
A general advantage of all spread spectrum communication systems is the minimization of the effects of narrow band interference. In particular, the direct sequence signal is despread or collapsed into a narrow band (with the overall band width being reduced by the direct sequence bandwidth) and the interference is spread out over the band. A relatively broad frequency band is required for frequency hopping, e.g., a bandwidth of 50 MHz is used for an instantaneous frequency signal band of 25 kHz. A frequency discriminator output is desired (which corresponds to or is representative of, a voltage-frequency plot) so as to determine, over a wide frequency range based on the voltage output, the frequency of the carrier at any instant of time. With this information, demodulation of the received signal can be carried out in a conventional manner.